1. Field of the Invention
The present invention relates to a roll eccentricity control system for a rolling apparatus, and more particularly to a roll eccentricity control system suitable for compensating a variation of thickness of a web material due to a roll eccentricity in a gauge control of a rolling apparatus for rolling a web material such as a hot rolling mill, a cold rolling mill or the like.
2. Description of the Prior Art
The rolls used in a rolling apparatus, such as a rolling mill for rolling a steel plate, do not have sections of exact true circle due to grinding accuracy (unevenness in grinding) in the manufacture of the rolls. In a rolling mill of a type in which the rolls are coupled to rotary shafts by keys, the keys are necessarily causes of roll eccentricity. Because of the roll eccentricity, a roll gap periodically varies at a period of one revolution of the roll. This causes a periodic variation of the thickness of the plate as rolled.
Several control methods have been proposed to prevent the variation of the plate thickness due to the roll eccentricity. One of well known methods is a programmed control method using off-line data, as disclosed in U.S. Pat. No. 4,038,848 issued on Aug. 2, 1977 to Ken Ichiryu et al and entitled "Method and Apparatus for Controlling Eccentricity of Rolls in Rolling Mill", in which a variation of a rolling load is detected while the rolls are rotated under a suitable rolling pressure but which no material to be rolled and stored as a data for indicating the roll eccentricity, and a variation of the roll gap due to the eccentricity during the rolling operation is compensated based on the store data.
The control unit which uses the programmed control method is generally useful, but has several problems in some cases. First, a roll rotation angular position sensor is required to store an eccentricity detection signal in correspondence to respective angular positions in one revolution of the roll. The sensor may be a pulse generator or selsyn generator. Such a rotation sensor is desired to be located as closely to a roll shaft as possible in order to enhance a detection accuracy. However, because of a mechanical structure around the roll, a circumstance (high temperature, oil mist, etc.) and a mounting structure of a rotation transmission mechanism, it is difficult to mount the sensor satisfactorily close to the roll shaft and hence difficult to detect the rotational position reliably with good reliability and good linearity in relationship between its output and the rotational position. Even if the close mounting is possible, it makes a roll exchange work which is frequently conducted troublesome. Further, in preparing the prestored compensation signal for thickness variation due to the roll eccentricity, it is necessary to warm up the rolls to a temperature near to a roll temperature which is encountered during the actual rolling process. Since such a roll temperature is relatively high, a time of 10-15 minutes is usually required to warm up the rolls and prepare the roll eccentricity compensation signal. Further, because of a difference from an actual rolling condition, an error necessarily occurs between the actual roll eccentricity and the stored data by various causes. Accordingly, there is a limit in the improvement of the gauge precision by this method.
A method for resolving the problems encountered in the above programmed control method has been proposed, for example, in U.S. Pat. No. 4,036,041 issued on July 19, 1977 to Ken Ichiryu et al and entitled "Gauge Control System for Rolling Mill", in which a variation of the plate thickness is measured by a load meter during the actual rolling operation, a resulting signal is processed by a correlation circuit or a filter circuit to produce an eccentricity component representing the roll eccentricity, and the plate thickness is corrected by controlling the rolling pressure in accordance with a rotational position detection signal.
In the control unit which uses this method, the measured signal from the load meter is statistically frequency-analyzed. Since a back-up roll is usually repeatedly used after scratches formed on the surface thereof during the rolling operation have been ground away, a diameter of an old roll may be 10-20% smaller than the diameter of a new one. As a result, a basic frequency of the frequency analysis varies and a center frequency of the filter circuit is shifted. Accordingly, an apparent detection gain is lowered and an exact roll eccentricity cannot be recognized. Further, the variation of the rolling load does not always exactlly represent the roll eccentricity. Especially, in case of the automatic gauge control of the gauge meter type in which the equivalent rigidity of the rolling stand may be greatly increased, the load signal includes components due to various factors other than the roll eccentricity. Therefore, there is a limit in controlling the roll eccentricity by this signal.